Make "." a builtin as well

Yeah, it's not going anywhere. This is one line in builtin.cpp vs 9
lines of script, most of which used to print an error that is never triggered.

share/config.fish

@@ -154,16 +154,6 @@ end
 # in UTF-8 (with non-ASCII characters).
 __fish_set_locale
 
-# "." alias for source; deprecated
-function . -d 'Evaluate a file (deprecated, use "source")' --no-scope-shadowing --wraps source
-    if [ (count $argv) -eq 0 ] && isatty 0
-        echo "source: using source via '.' is deprecated, and stdin doesn't work."\n"Did you mean 'source' or './'?" >&2
-        return 1
-    else
-        source $argv
-    end
-end
-
 # Upgrade pre-existing abbreviations from the old "key=value" to the new "key value" syntax.
 # This needs to be in share/config.fish because __fish_config_interactive is called after sourcing
 # config.fish, which might contain abbr calls.